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While reading on block ciphers and DES I read that two-round Feistel network is not a secure PRP? Is there any easy to understand proof to explain the intuition behind this statement. I did search around and also reviewed this question but wasn't really able to understand why it isn't secure PRP. Any help would be much appreciated!

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I read that two-round Feistel network is not a secure PRP

That's easily seen:

                                    two-rounds Feistel cipher

It holds $P_L\oplus C_L=F_0(P_R)$. That implies a distinguishable property: for any fixed $P_R$ and whatever the round function $F_0$, when we flip bit(s) in $P_L$, that flips the corresponding bit(s) in $C_L$ and leaves the other bit(s) in $C_L$ unchanged.

That property allows a break under Chosen Plaintext Attack.

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  • $\begingroup$ Perfect simple explanation that I was looking for! Thanks a lot! $\endgroup$
    – Alex
    Oct 25, 2020 at 20:51
  • $\begingroup$ Umm... Isn't that 1-round Feistel? $\endgroup$
    – hola
    May 30, 2021 at 6:26
  • $\begingroup$ @hola: the common way of counting rounds in Feistel ciphers is counting how many round functions there are. In the picture, there are two. $\endgroup$
    – fgrieu
    May 30, 2021 at 7:46

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